Only invalidate row height cache when a row is really expanded

It's wasteful to do it if it's already expanded or won't be expanded
because the EXPANDING event is vetoed.

src/generic/datavgen.cpp

@@ -3919,13 +3919,6 @@ void wxDataViewMainWindow::Expand( unsigned int row )
     if (!node->HasChildren())
         return;
 
-    if ( m_rowHeightCache )
-    {
-        // Expand makes new rows visible thus we invalidates all following
-        // rows in the height cache
-        m_rowHeightCache->Remove(row);
-    }
-
     if (!node->IsOpen())
     {
         if ( !SendExpanderEvent(wxEVT_DATAVIEW_ITEM_EXPANDING, node->GetItem()) )
@@ -3934,6 +3927,13 @@ void wxDataViewMainWindow::Expand( unsigned int row )
             return;
         }
 
+        if ( m_rowHeightCache )
+        {
+            // Expand makes new rows visible thus we invalidates all following
+            // rows in the height cache
+            m_rowHeightCache->Remove(row);
+        }
+
         node->ToggleOpen(this);
 
         // build the children of current node